Bearing seals for various different applications must be designed to meet various operating requirements such as pressure differential to be applied across the seal, intrusion by mechanical and/or chemical contaminants, operating temperature levels, and the like. The bearing seals used in rock bits, and particularly rock bits for drilling deep oils wells, must operate under relatively unique conditions.
The conventional design of rock bit bearing seals avoids, insofar as feasible, the application of any pressure differential across the seal. This is achieved by utilizing in the internal lubrication system of the rock bit a pressure sensing and pressure equalizing system whose purpose is to equalize the internal pressure of the lubricant with the ambient pressure of drilling mud or other drilling fluid that occupies the well bore. As a result of these conventional pressure equalizing systems the pressure differential across the bearing seal is limited, hopefully, to perhaps one pound per square inch.
Nevertheless, rock bit bearing seals operate under extremely demanding conditions. Some type of flexible sealing medium is necessarily utilized in the structure of the seal. As the depth of the well bore increases, the ambient pressure of the drilling fluid also increases in substantially linear fashion and typically reaches a pressure level of many thousands of pounds per square inch. At the same time the necessary rotation of the rock bit, and pulverization of the rock material which it encounters, engender large amounts of heat which is not easily dissipated. The rock bit bearing seal, therefore, operates under levels of both pressure and temperature which are extremely high compared to any other types of rotating seal applications.
There is in addition, a severe problem of contamination of rock bit bearing seals. The continuous circulation of the drilling fluid carries away many small or minute particles of pulverized rock which are entrained within the drilling fluid, and the exterior side of the bearing seal is continuously subjected to these particles. Many particles enter the seal with resultant damage which drastically limits the useful life of the seal and hence of the entire rock bit.